FIG. 7 is a graph showing a relation between an amount of flow of cooling water and an engine rotational speed for a conventional liquid-cooling device. Conventionally, a mechanically operated pump for circulating the cooling water is operated by receiving a driving force from the engine. Therefore, the amount of the flow of the cooling water is proportional to the rotational speed of the engine, as shown in FIG. 7.
However, such operation of the pump is not optimum for cooling the engine. Some cooling devices have been proposed in the art to improve the operation of the pump. For example, in Japanese Patent No. 2767995, an additional valve and an electrically operated pump are provided to form a new path of the liquid flow in the liquid-cooling device. In Japanese Patent Publication No. 2000-45774, a mechanically operated pump is replaced by an electrically operated pump.
Furthermore, in Japanese Patent Publication No. H8-128559, a flow amount adjusting valve is disclosed, wherein temperature of the cooling water is controlled at a higher value when an engine load is low, to improve fuel consumption ratio.
The mechanically operated pump is installed at a cylinder block of the engine, because the pump is driven by the engine through a driving belt. The cooling water is designed to enter the engine from the cylinder block and goes to a cylinder head through the cylinder block, in order to remove air bubbles produced in the cooling water. When the cooling water flows into the cylinder head from the cylinder block, the cooling water passes through a hole formed in a gasket between the cylinder head and the cylinder block. The hole narrows and accelerates the flow of the cooling water. The accelerated flow cools down the cylinder head efficiently. The mechanical type pump is designed to discharge the cooling water, a flow rate of which is over 100 l/min at a high engine rotational speed of 5,000 to 6,000 rpm. Namely, the mechanical type pump is designed to meet a requirement at the high engine rotational speed, under an assumption that a high engine load is equal to the high engine rotational speed.
As a result, an excessive amount of the cooling water flows through the engine at a low load engine operation, such as an ordinary running of a vehicle, causing a problem that the excessive amount of the cooling water may deteriorate a warming-up performance for the engine. Furthermore, since the cooling water flows in an engine cooling water circuit having a high flow resistance, due to the hole formed in the gasket, a higher pump driving force is required for the engine, causing a problem that a fuel consumption ration is decreased. Furthermore, since the mechanical type pump supplies the cooling water, a flow rate of which is only in proportion to the engine rotational speed, it is rather difficult to quickly cool down the cylinder head portion when the engine load is rapidly increased due to, for example, a rapid acceleration of a vehicle. As a result, it may cause a problem of engine knocking.